1. Field of the Invention
This invention relates to electronic heart pacing apparatus and in particular to cautery protection circuits associated therewith.
2. State of the Prior Art
Heart pacers such as that described in U.S. Pat. No. 3,057,356 issued in the name of Wilson Greatbatch and assigned to the assignee of this invention, are known for providing electrical stimulus to the heart whereby it is contracted at a desired rate in the order of 72 beats per minute. Such a heart pacemaker is capable of being implanted within the human body and operative in such an environment for long periods of time. Typically, such pacemakers are implanted in the pectorial region or in the abdominal region of the patient by a surgical procedure, whereby an incision is made in such region and the pacemaker with its own internal power supply, is inserted within the patient's body.
In FIG. 1, there is shown an output portion of an electronic heart pacemaker of the prior art. The output circuitry includes a transistor Q12' which is periodically turned on and off at a rate corresponding to that with which the patient's heart is to be stimulated, e.g. 72 beats per minute, and for a duration sufficient to stimulate the patient's heart. The base and collector electrodes are connected respectively to suitable biasing resistor R19', and capacitor C8' and charging resistor R20', and the output is coupled from the collector of the transistor Q12' by a suitable capacitor C8'. A zener diode CR10' is connected across the output of the circuit to provide defibrillation protection. In the normal functioning of the heart, an electrical charge is established across the muscle tissue of the heart, i.e. polarization, and is subsequently discharged, i.e., depolarization. In fibrillation, there are many origins of depolarization which interact with each other and, as a result, the heart assumes a random motion, whereby little if any blood is circulated in the arterial system of the patient. To reinitiate the normal activity of the heart, a defibrillation pulse of relatively large amplitude is applied across the patient's heart. Typically, a pair of paddles (electrodes) is placed on each side of the patient's chest, whereby the defibrillation pulse is applied to his heart to reinitiate the normal rhythmic operation of his heart. The defibrillation pulse as seen by the output portion of the heart pacemaker circuit is in the order of 1500V. It is expected that such a large voltage could easily damage if not destroy the circuit elements of the circuit unless otherwise protected. To prevent this, the zener diode CR10' is inserted across the output, thereby limiting the voltage applied to the pacemaker circuitry to a safe level, e.g. 8V.
The surgical procedure for implanting or removing the heart pacemaker into the body of the patient may involve cauterizing the incision made for the pacemaker pocket, thereby sealing off the small blood vessels surrounding the pocket. In FIG. 2, there is shown a patient with a heart pacemaker 10 implanted therein and the use of a cautery electrode 12 for cauterizing the pacemaker incision. Typically, a cautery unit such as the Bovie Electrosurgical Unit applies an electrical signal such as shown in FIG. 3 to the electrode 12. The high frequency signal has a "damped" waveform; the term "damped" means that the current is in pulses which start with a maximum amplitude and decrease in amplitude at a logrithmic rate. These groups of pulses are sometimes referred to as wave trains and the number of these wave trains occurring per second is called the wave train frequency. The rate at which the pulses occur in each wave train (the number per second) denotes the frequency of the unit, e.g. 500 to 800 kilocycles per second. In the following table, I show values for several characteristics of the two basic currents. The values are approximate, but at the same time representative of current practice.
__________________________________________________________________________ OSCILLATING WAVE TRAIN PEAK OUTPUT MAXIMUM CURRENT FREQUENCY FREQUENCY VOLTAGE (NO LOAD) OUTPUT __________________________________________________________________________ Cutting 500-800 KHz 30000-50000/sec 3000-3500 volts 250 watts Coagulating 500-800 KHz 10000-15000/sec 5000-7500 volts 150 watts __________________________________________________________________________
As shown in FIG. 2, an electric field 18 is established between the cautery electrode or forcep 12 and a cautery ground plate 16 disposed against the patient's buttocks. As shown in FIG. 2, the artificial heart pulse generator 10 and its electrode 14 are disposed in the path of the field 18, whereby a signal is readily induced into the output portion of the heart pacemaker circuitry.
In the above-referenced patent application of Joseph A. Ballis, there is described a cautery protection circuit for use with a heart pacemaker, whereby the effects of signals induced within the output circuit of the heart pacemaker due to an electric field established by cautery procedures, are minimized. As explained in the above-identified patent application, cautery procedures are particularly prone to induce unsymmetrical signals upon the output circuit of unipolar-type heart pacemakers, and such unsymmetrical signals may stimulate the patient's heart into fibrillation. There is shown in FIG. 4 the cautery protection, output circuit as described in the above-identified patent application of Joseph A. Ballis, wherein there is included having an output transistor Q12, and a diode CR14 disposed across the output transistor Q12 in a manner to prevent the output transistor Q12 from being operated in a non-symmetrical manner and tending to render the pacemaker's conduction of cautery induced signals symmetrical, thus reducing the risk of stimulating the patient's heart into fibrillation. Further, the capacitor C8 is connected across the output of the transistor Q12 and is selected to be of such a value to provide a low impedance path to those frequencies as would be induced in the output circuit by the cautery-produced field. Further, zener diodes CR10 and CR12 are connected in-series and in an opposing manner, whereby the heart pacemaker circuitry is protected with respect to relatively large defibrillation pulses. A capacitor C12 is connected in-parallel across the output of the transistor Q12 to detune the resonant circuit formed by the junction capacitances of the diodes CR14, CR10 and CR12, and an inductance L1 disposed in-series between the output circuit and the lead directed therefrom to the patient's heart. The inductance L1 is used as a current-limiting device to reduce the current flow of the high-frequency components that otherwise would be induced therein by cautery procedures.
Typically, the inductor L1 as shown in FIG. 4 is an axial lead, epoxy-molded type inductor having a relatively high DC resistance in the order of approximately 10 ohms. If the impedance of the leads and of their coupling to the heart is in the order of 500 ohms, such an impedance load is not a significant factor. However, if low-threshold leads are used so that the impedance offered by the leads and the heart is in the order of 100 ohms, the resistance offered by such epoxy-molded inductors L1 presents a significant additional load to the heart pacemaker circuit, attenuating its output pulses significantly and placing an undue power drain upon its batteries.
Further, such an inductor typically has an undesired shaping effect upon the pacemaker stimulating signal, having a normally very rapid rise time in the order of 1 .mu.sec, whereby its pulse rise time is increased by a factor of approximately 8-10. In addition, the inductance L1 may also tend to impose a ringing upon the trailing edge of the pacemaker pulse. This undesirable distortion of the pacemaker pulse may possibly prevent the subsequent monitoring of a heart pacemaker with the use of commonly-available radios. In this regard, it is a common practice for doctors, as well as patients, to monitor the operation of an implanted pacemaker by use of a transistorized radio held next to the patient's body and tuned to a spot on the dial where a radio station normally would not be heard. Such a disposed radio picks up the pacemaker pulses to provide a sound from its speaker in the nature of a "click" upon the occurrence of each pacemaker pulse. The number of such pulses within a minute may be counted to provide an indication of battery life and pacemaker operation. Thus, if the pacemaker output is significantly distorted, the use of such a simple means to monitor the operation of an implanted pacemaker may no longer be available.